I was looking around for jQuery editors the past couple of days and also came across jTagEditor [1]. This is just a tag inserter, and has some shortcomings even as that - but I like the way it supports pluggable markup alongside xhtml (the demo has bbcode, wiki syntax and textile implemented).

Wysiwym along the lines of wymeditor (I don't think wyg in a textarea is ever going to be possible with 91 contrib filter modules - how do manage with view insert tags and the rest?) + pluggable input filters/buttons + live previews[2] seems like an ideal option either in core or contrib.

1. http://www.jaysalvat.com/jquery/jtageditor/
2.http://drupal.org/project/live (and see it in action on http://groups.drupal.org)